Technology for positioning mobile radio terminals using the signals received from one or more transmitters has been widely used for many years. Such systems include terrestrial networks of transmitters (e.g. Loran) and satellite constellations (e.g. GPS, GLONASS and Galileo) deployed specifically for the purpose of locating the receiver, as well as methods that use general-purpose radio networks such as cellular mobile telephone networks (e.g. WO-A-97-11384) or TV and radio transmitter networks. (e.g. EP-A-0303371).
Within a cellular mobile telephone network, for example, the position of the terminal may be based on the identity of the serving cell, augmented by information such as the signal transmission time delay between the serving transmitter and terminal, the strengths of signals received from the serving and neighbouring transmitters, or the azimuth angles of incidence of received signals. An improved position may be obtained using the observed time difference of arrival (OTDA) of signals received at the terminal from two or more transmission sources.
OTDA methods give good position accuracy using only the signals available within the cellular radio network. However, they require the precise transmission time offsets between the transmitters of the network to be determined in order to solve the positioning equations. This can be done using location measurement units (LMUs) having additional receivers. LMUs are placed at known locations so that their OTDA measurements can be converted directly into a network timing model (see for example WO-A-00-73813).
Alternatively a technique (see WO-A-00-73814) may be used in which measurements of signals from a number of geographically disparate transmitters at known positions made, for example, by two geographically disparate terminals at unknown positions, may be used to compute both the positions of the terminals and all the timing offsets between the measured transmitters, without the need for LMUs.
A satellite positioning system, such as GPS, gives an accurate result provided that the receiver can receive sufficient satellite signals. The satellite signals are related to a common time-base of a globally defined standard time, e.g. GPS Time or Universal Coordinated Time, UTC. For example, within GPS, each satellite in the constellation has a stable atomic clock whose time is continuously measured and compared with a single reference clock located on the ground. The time of each satellite clock is steered towards alignment with the reference clock and a parametric model is derived which describes the difference in time between the two clocks. The parameters (three for GPS) are up-loaded to the satellite and broadcast by the satellite as the clock correction parameters. This has the effect, after making corrections based on the parameters, of aligning the satellite clock closely with the ground-based reference clock.
Satellite positioning systems work well in situations where the receiver's antenna has clear sight of the sky, but they work poorly, or not at all, inside buildings or when the view of the sky is obscured. The current invention provides a solution to this problem by using both satellite and network measurements to provide a robust location determining system. Reversionary modes are identified which use combinations of satellite and cellular positioning, either alone or in combination, by which a continuous location solution is available when either the satellite signals are obscured or there are insufficient terrestrial signals available.
The performance of a satellite positioning system (SPS) can be improved by supplying the satellite positioning receiver with assistance data. A requirement in the provision of assistance to satellite positioning receivers carried in mobile terminals is the communications overhead associated with the assistance data. The invention provides a means for significant reduction in the quantity of data which has to be transferred, releasing communications capacity for other uses.
The protection of the privacy of the user of a mobile terminal is considered of great importance. Consequently, the transmission of location information over a communications network can be sensitive. A feature of this invention is that neither the absolute location of the terminal, nor an approximation thereof, is transmitted over the data link.
A further benefit of the invention includes the use of cellular location to provide for the intermittent or continual pre-positioning of the satellite receiver code-phase search process whilst satellite signals are too attenuated to provide this function alone with adequate reliability. The provision of position aiding may benefit in ways other than improved accuracy, such as faster time to first fix, longer battery life or lower communications usage. A yet further benefit of the invention provides for the limitation of the search range for the received satellite code signal alignment in the satellite receiver. This allows the use of less-complex silicon chips for the satellite positioning system.
There is a requirement to determine the number of code intervals between the satellite and the receiver in the transient phase of a satellite positioning system location calculation between first measurements and stable solution. This is commonly known as the integer ambiguity problem. The positioning information derived from the cellular network in the present invention is used to determine uniquely the number of code intervals between each satellite and the receiver, thereby avoiding this problem.